One of the critical features in machine guns is the so-called "headspace" which is the distance between the front face of the bolt and a defined location inside the breech and as a rule, correct headspace must be observed at all times. Incorrect headspace may cause, on one hand, firing failure and on the other hand, if priming of the cartridge is performed while the cartridge is not snugly fitted inside the breech, part of the energy liberated by the explosion of the propellant will dissipate sideways and rearwards, resulting in reduction of the force acting on the bullet emerging from the barrel and consequently of its range and accuracy. Additionally and more importantly, the sidewards and rearwards expanding explosion waves may damage the rifle and sometimes also cause injuries to the gunner.
Machine guns in general, and light machine guns in particular, are designed for high speed, sustained firing. During such firing the barrel heats up considerably which may cause bullet cook-off and at times even irreversible damage, such as the formation of cracks and blow-up. In order to avoid such damages, it is necessary to replace the barrel from time to time during prolonged firing.
A major problem in a machine gun with a replaceable barrel, is the need to ensure that the headspace always remains the same. In some machine guns the distance between the bolt and the barrel is adjusted shortly before firing with the aid of special gauges designed therefor. Obviously such adjustment under field conditions is inconvenient and not within the capability of every soldier. Moreover, experience shows that such field adjustment, even if adequately performed, is not always satisfactory and additionally may change during firing.
In view of these difficulties it is usually the practice today to provide each individual machine gun with a pair of interchangeable barrels, factory adjusted to fit a given machine gun, in order to ensure the correct headspace. However, even this solution is not quite satisfactory and experience shows that under operational stresses mistakes do occur and a barrel intended for use with one machine gun is mistakenly attached to another.
It is an object of the present invention to provide a barrel locking mechanism for a machine gun which ensures accurate adjustment of the barrel inside the body to achieve the desired headspace. It is a further object of the present invention to so design such locking mechanisms that barrels mass-produced for a particular type of machine gun are interchangeable without any restriction.